PREFIRE PLANT COMMUNITY AND FUELS:
The vegetation was desert grassland dominated by "nearly pure" stands of tobosa and buffalo grass. Honey mesquite were "thick" and tree cholla and tulip pricklypear were scattered throughout both stands. The entire area had been aerially sprayed with herbicide (2,4,5-T) 7 years prior to the study (1963), achieving 95% top-kill of honey mesquite but <5% honey mesquite mortality. The year prior to the fire (1969), when honey mesquite was 3 to 6 feet tall, a portion of the study area was chained. Few trees were killed by chaining. In chained areas, honey mesquite trees were lying on the ground next to their bases [2,3].

The study site is classified in the following plant community and probably historically experienced the fire regime described below:

Fire regime information on the vegetation community studied in this Research Project Summary.
Fire regime characteristics are taken from the
LANDFIRE Rapid Assessment Vegetation Model [5]. This vegetation model was developed by local
experts using available literature and expert opinion as documented in the PDF file linked from the Potential Natural Vegetation Group listed below. Cells are blank where information is not available in the Rapid Assessment Vegetation Model.

*Fire Severities:Replacement=Any fire that causes greater than 75% top removal of a
vegetation-fuel type, resulting in general replacement of existing vegetation; may or may not
cause a lethal effect on the plants.Mixed=Any fire burning more than 5% of an area that does not qualify as a replacement, surface, or low-severity fire; includes mosaic and other fires that are intermediate in effects.Surface or low=Any fire that causes less than 25% upper layer replacement and/or
removal in a vegetation-fuel class but burns 5% or more of the area
[1,4].

FIRE DESCRIPTION:
Objectives of the study were to determine 1) postfire mortality of honey mesquite, tree cholla, and tulip pricklypear the first 2 growing seasons after a March prescribed fire; 2) the percent consumption by fire of honey mesquite logs previously chained; 3) the effects of prescribed fire on the composition and production of grasses and forbs during the 1st postfire growing season; and 4) cattle utilization of tobosa and buffalo grass in burned and unburned plant communities during the 1st postfire growing season. They also examined the combined effects of chaining and prescribed fire on honey mesquite, tree cholla, and tulip pricklypear during postfire growing seasons 1 and 2.

A headfire was started at 3:30 PM on 23 March 1970, and was completed by 4:30 PM. Three hundred acres were burned. The fire was spotty and incomplete due to insufficient fuels in some areas, especially where buffalo grass had been heavily grazed.

Average annual rainfall in the region was 19 inches. A "mild" drought occurred during postfire year 1, when rainfall in the area ranged from 13 to 15 inches. Vegetation green-up occurred in March prior to the fire, which produced "excellent" soil moisture conditions [2]. Precipitation in the area from 1 to 23 March ranged from 2.6 to 3.3 inches [3]. Five days following the fire, precipitation was 0.3 inches. Between 1 May and 15 June, precipitation was 2.8 inches [2]. Thereafter, precipitation was "light" and little vegetation growth occurred [3]. Vegetation green-up occurred again in September when precipitation in the area ranged from 2.3 to 6.4 inches [2]. Precipitation data was not provided for postfire year 2.

FIRE EFFECTS ON PLANT COMMUNITY AND FUELS:Honey mesquite: No honey mesquite trees were killed by fire. Most were top-killed but sprouted soon after fire. Six months after the fire, sprouts were up to 4 feet tall.

Consumption of honey mesquite logs by fire was greater for large (2-6 inches) than small (<2 inches) logs in chained tobosa and buffalo grass stands due to the loose bark on large logs giving them a greater surface area to volume ratio of exposed fuel. A greater proportion of logs was consumed in tobosa stands than buffalo grass stands because of higher fine fuel loads in tobosa stands. Log consumption increased linearly as fine fuel loads increased: when <1,200 pounds/acre of fine fuels was present, no logs were consumed; when >1,600 pounds/acre of fine fuels was present, >50% of logs were consumed [2,3].

Tree cholla: During the 1st and 2nd postfire growing seasons, a higher percentage of tree cholla plants had been killed by fire in tobosa than buffalo grass stands in both chained and unchained areas due to higher burning temperatures in tobosa stands (Table 1) [2,3]. During the 1st postfire growing season, many tree cholla plants had sprouted on both sites, averaging 5 sprouts/plant [2]. Tree cholla mortality increased substantially in all stands during the 2nd postfire growing season, although statistical comparisons were not provided. This result was attributed to drought and insect and small mammal herbivory. Short tree cholla plants always had greater mortality than tall tree cholla plants, although differences were significant only in unchained areas. The authors suggested that mortality was statistically similar between short and tall tree cholla plants in chained areas because most tree cholla plants were lying close to the ground [2,3].

Table 1: Mortality (%) of short (<1 foot tall) and tall (>1 foot tall) tree cholla after a March prescribed fire in chained and unchained stands in tobosa and buffalo grass communities. No tree cholla mortality was observed in unchained and unburned control stands. All areas were herbicide-sprayed 7 years prior to the fire, and some were chained 1 year prior to the fire [3].

Community

Tobosa

Buffalo grass

Sample date

6 months after fire
(Sept 1970)

16 months after fire
(July 1971)

6 months after fire
(Sept 1970)

16 months after fire
(July 1971)

Chained and burned

Short

32

93

25

75

Tall

20

80

18

50

Unchained and burned

Short

33*

81*

15*

48*

Tall

6

45

0

4

*Short and tall plant mortality were significantly different at P≤0.05.

Tulip pricklypear: First postfire growing season mortality of tulip pricklypear was similar in chained and unchained tobosa stands. In buffalo grass stands, its mortality was greater in chained than unchained stands because most pricklypear plants were lying close to the ground in chained areas and because patches of buffalo grass were denser, and thus fine fuel loads were greater, in chained than unchained stands. During the 1st postfire growing season, tulip pricklypear mortality in tobosa and buffalo grass stands was higher for small plants than large plants (Table 2) [2,3].

During the 2nd postfire growing season, tulip pricklypear mortality increased substantially in both stands, with >50% killed in all treated stands. Its mortality in tobosa stands was similar in chained and unchained stands. In buffalo grass stands, mortality remained higher in chained than unchained stands. Increased mortality during the 2nd postfire growing season was attributed to drought and insect and small mammal herbivory. There were no differences in mortality between small and large plants by the end of the 2nd growing season [2,3].

Table 2: Mortality (%) of small (<10 pads) and large (>10 pads) tulip pricklypear after a March prescribed fire in chained and unchained tobosa and buffalo grass communities. No tulip pricklypear mortality was observed in unchained and unburned control stands. All areas were herbicide-sprayed 7 years prior to the fire, and some were chained 1 year prior to the fire [3].

Community

Tobosa

Buffalo grass

Sample date

6 months after fire
(Sept 1970)

16 months after fire
(July 1971)

6 months after fire
(Sept 1970)

16 months after fire
(July 1971)

Chained and burned

Small

50*

89

48*

85

Large

24

89

21

74

Unchained and burned

Small

54*

86

23*

53

Large

11

79

6

53

*Small and large plant mortality were significantly different at P≤0.05.

Tobosa: Grass production in tobosa stands increased 3-fold the 1st growing season after fire (Table 3). Tobosa's unpalatable old growth was removed by burning [2,3]. New growth following fire was tender and succulent and cattle preferred it to buffalo grass 3 months after fire. Total tobosa grazed by cattle on the burned area 3 months after fire was 2,388 pounds/acre, while only 371 pounds/acre were grazed on the control. Five months after fire, when tobosa was dry, cattle preferred buffalo grass to tobosa and grazed only small amounts of tobosa on burned and unburned control stands. Seven months after fire, after vegetation green-up, total tobosa grazed by cattle on the burned area was 431 pounds/acre, while only 122 pounds/acre was grazed on the unburned control [2].

Buffalo grass: The fire had no effect on grass production in buffalo grass stands the 1st growing season after fire (Table 3) [3]. Three months after fire, cattle preferred tobosa to buffalo grass. Total buffalo grass grazed by cattle on the burned area 3 months after fire was 873 pounds/acre, while only 473 pounds/acre were grazed on the unburned control. Five months after fire, when buffalo grass was dry, cattle preferred buffalo grass to tobosa but grazed only small amounts of buffalo grass on burned and unburned control stands because cattle had shifted to other grass species. Seven months after fire, after vegetation green-up, total buffalo grass grazed by cattle on the burned area was 201 pounds/acre, while 182 pounds/acre was grazed on the control [3].

Forbs: Production (pounds/acre) of several annual herbs, including prairie broomweed and horsetail conyza tended to be lower on burned than unburned areas during the 1st growing season. False mesquite production was higher on burned than unburned areas. Warty euphorbia, western ragweed, and evax occurred on unburned areas but were absent from burned areas (Table 3) [2,3].

Table 3. Herbaceous plant production (pounds/acre) the 1st growing season following a March prescribed fire in chained, unchained, and unchained and unburned control stands in tobosa and buffalo grass communities. All areas were herbicide-sprayed 7 years prior to the fire, and some were chained 1 year prior to the fire [3].

Species

Chained and burned

Unchained and burned

Unchained and unburned control

Tobosa

Buffalo grass

Tobosa

Buffalo grass

Tobosa

Buffalo grass

Grasses

3,468

1,617

3,058

1,974

1,974

1,971

Forbs

False mesquite

79

126

120

102

28

53

Prairie broomweed

22

18

15

19

79

69

Silverleaf nightshade

1

4

9

1

21

2

Warty euphorbia

0

0

0

0

6

3

Western ragweed

0

0

0

0

18

0

Horsetail conyza

0

4

2

2

1

59

Evax

0

0

0

0

2

3

Wooly plantago

0

2

1

0

1

4

Other

23

42

18

10

30

20

FIRE MANAGEMENT IMPLICATIONS:
The authors concluded that spring prescribed fire in tobosa and buffalo grass communities may help control undesirable herbaceous annuals while increasing tobosa production [3]. One objective of this study was to determine the effects of prescribed fire and chaining on honey mesquite, tree cholla, and tulip pricklypear the first 2 growing seasons after fire. Burning did not kill any honey mesquite trees in any treatment during the first 2 postfire growing seasons. Postfire tree cholla mortality was highest in tobosa stands, where fine fuel loads were greatest. Tree cholla mortality was generally greater in chained than unchained stands. More than half of all tulip pricklypear plants had died in all treated areas by the end of the 2nd postfire growing season. In buffalo grass stands, chaining appeared to increase tulip pricklypear postfire mortality.

Other objectives of this study were to determine the effects of prescribed fire on the composition and production of grasses and forbs and to compare cattle utilization of tobosa and buffalo grass in burned and unburned plant communities during the 1st postfire growing season. Tobosa production increased following fire. Cattle consumed 2,016 pounds/acre more tobosa on burned areas than on an unburned control area. Buffalo grass production was unchanged after fire. False mesquite production increased after burning, but prairie broomweed and horsetail conyza production tended to decrease [2,3].

APPENDIX: SPECIES INCLUDED IN THIS SUMMARY
This Research Project Summary contains fire effects and/or fire response information on the following species. For further information, follow the highlighted links to the FEIS reviews of those
taxa.